This invention relates to a pressure-sensitive recording sheet, and more specifically, to a color-developing sheet for pressure-sensitive recording sheets which contains a novel color developing agent.
Pressure-sensitive recording sheets are also known as carbonless copying paper. They produce a color upon the application of a mechanical or impact pressure by writing or by pounding a typewriter, thus permitting duplication of several copies simultaneously. The color is based on a color forming reaction between an electron-donating colorless dye and an electron-accepting color developer.
The structure of a pressure-sensitive recording sheet and the mechanism of color formation are illustrated generally with reference to the accompanying drawing. The back surface of each of a top (CB: Coated Back) sheet 1 and a middle (CFB: Coated Front and Back) sheet 2 is coated with microcapsules 4 having a diameter of several microns to ten and several microns and composed of a shell of a polymeric film such as gelatin and a solution of a colorless color-forming pressure-sensitive dye in an involatile oil enclosed therein. The surface of each of the middle CFB sheet 2 and the bottom (CF: Coated Front) sheet 3 is coated with a layer 5 containing a color developing agent having the property of reacting with the colorless dye upon contact therewith and thus producing a color. A sheet coated with a coating composition containing a color developer, such as the CFB sheet and the CF sheet, is called a color-developing sheet. When a localized pressure is applied by a writing instrument 6 (or a typewriter or the like) to a pressure-sensitive recording sheet composed of a multiply structure of CB sheet 1, CFB sheet 2 and CF sheet 3 so that the microcapsule-coated surface faces the surface coated with the color developer-containing layer 5, the microcapsules 4 under the applied pressure break and the solution of the colorless dye moves to the color developer-containing layer 5. Thus, the dye reacts with the color developer to from a colored image 7 in the desired pattern of recording. In the pressure-sensitive recording sheet illustrated in the drawing, only the CFB sheet is interposed between the CB sheet 1 and the CF sheet 3, but if desired, two or more CFB sheets may be interposed.
Conventionally known electron-accepting color developing agents include (1) inorganic solid acids such as acid clay (Fuller's earth) or attapulgite, (2) substituted phenols and diphenols, (3) p-substituted phenol formaldehyde polymers, and (4) metal salts of aromatic carboxylic acids. These color-developing agents, however, are not entirely satisfactory. For example, the inorganic solid acids adsorb gases or moisture in the air and cause yellowing of the sheet or are deteriorated in color-forming property. With the substituted phenols and diphenols, the ultimate density of color is unsufficient. The p-substituted phenol formaldehyde polymers (for example, p-phenyl phenol novolak resin) have superior color-forming properties, but have the defect that the coated sheet undergoes yellowing upon exposure to sunlight. The aromatic carboxylic acid metal salts are superior in color-forming ability, light fastness of the colored image and resistance to yellowing under light, but their water resistance and plasticizer resistance are not entirely satisfactory.
It is a first object of this invention to provide a color-developing sheet which exhibits an equivalent or higher color-developing ability to or than developing sheets containing conventional inorganic solid acids or p-phenylphenol novolak resin as a color developing agent, and which give colors having superior water resistance and plasticizer resistance.
A second object of this invention is to provide a color developing sheet which gives a colored image having superior light fastness and undergoing little decrease in density with time, and whose tendency to yellowing under sunlight or the like is drastically reduced.
A third object of this invention is to provide a color-developing sheet which lends itself to very advantageous handling and storage.
According to this invention, there is provided a color-developing sheet for pressure-sensitive recording sheets, said color-developing sheet comprising as a color-developing agent at least one 2,2'-bisphenolsulfone nickel salt selected from compounds of the following formula ##STR3## wherein R.sub.3, R.sub.4, R.sub.5, R.sub.6, R.sub.7 and R.sub.8 are identical or different, and represent a hydrogen atom, a halogen atom, an alkyl group containing 1 to 10 carbon atoms, a cycloalkyl group containing 3 to 10 carbon atoms, an aralkyl group containing 7 to 11 carbon atoms, or a phenyl group, or a mixture of at least one 2,2'-bisphenolsulfone nickel salt of the above formulae (II) and (III) and at least one 2,2'-bisphenolsulfone zinc salt of the following formula ##STR4## wherein R.sub.1 and R.sub.2 are the same as R.sub.3 through R.sub.8 defined above.
In a preferred embodiment, the present invention provides a color-developing sheet for pressure-sensitive recording sheets, said color-developing sheet comprising both the aforesaid 2,2'-bisphenolsulfone metal salt and at least one polyvalent metal compound selected from the group consisting of oxides, hydroxides and carbonates of zinc, magnesium, aluminum, lead, titanium, calcium, cobalt, nickel, manganese and barium.
The 2,2'-bisphenolsulfone metal salts of general formulae (I), (II) and (III) are known compounds. However, it has not been known that these compounds form a color upon contact with a colorless pressure-sensitive dye to form a colored image having superior fastness characteristics, and are thus suitable as a color-developing agent for pressure-sensitive recording sheets. In addition, it has been difficult to anticipate the suitability of the 2,2'-bisphenolsulfone metal salts as a color-developing agent.
Free bisphenolsulfone, a precursor for the metal salts of formulae (I), (II) and (III), has been found to have little or no color-forming ability when used as a color-developing agent for pressure-sensitive recording sheets.
The color-developing sheet of this invention containing the metal salt of formulae (I), (II) or (III) has an equivalent or greater color-forming ability to or than color-developing sheets containing inorganic solid acids or p-phenyl phenol novolak resin as a color-developing agent, and the image formed has good resistance to fading under the action or water, plasticizers, light, etc. In particular, a color-developing sheet containing a nickel salt of 2,2'-bisphenolsulfone as a color-developing agent gives an image which has very good resistance to light (i.e. very good light fastness). Thus, this sheet is better than a sheet comprising as a color-developing agent an aromatic carboxylic acid metal salt which proves to be relatively good among the color-developing agents heretofore suggested. The color-developing sheet of this invention also has much improved resistance to yellowing under exposure of sunlight or the like, and lends itself to very advantageous handling and storage.